The Space Shuttle external fuel tank is a tank 271/2 feet in diameter, 153 feet long, made from 0.080 inch thick lithium-aluminum alloy. These tanks are made by welding together several cylindrical sections, together with hemispherical end sections. Welding of the seams between sections is rendered difficult by the need to align the edges being welded to within 0.003 inch during the welding operation, and because of the need for essentially perfect, nonporous, void- and contaminant-free welds.
In order to position the sections of the tank preparatory for welding, the various tank sections are mounted on an internal mandrel having a horizontal axis. The mandrel is rotatable about its axis, so that the tank sections can be rotated to present all portions of one of the circumferential butt joints to a welder. The thin material of the tank sections would sag to a noncircular form, and cause mismatch and peaking between the surfaces, if it were not supported. Support is provided to the edges of each of the adjacent sections being welded by mutually adjacent portions of the mandrel. The portion of the mandrel adjacent to, and in actual contact with the tank section, is made up of a plurality of radially extending screws. These screws allow the "shape" of the mandrel to be adjusted slightly before welding, so that the tank sections are circular to within the desired tolerance, and so that the abutting edges which are to be welded are within the desired 0.003 inch radial tolerance. When the welding is finished, the internal mandrel is disassembled and removed through an access port.
The aluminum-lithium alloy from which the tank is made tends to oxidize at the temperatures necessary for welding. This oxidation is disadvantageous, in that it can result in porosity, voids, inclusions, or other defects in the welded seam. Improved welding is desired.